Titanium or titanium-alloy substrate for magnetic-recording media

ABSTRACT

An improved magnetic-recording disk and a process for manufacturing magnetic-recording disks are disclosed. Precision cold-rolled titanium or titanium alloy is the substrate for a magnetic-recording disk. The surface of the substrate may be hardened by plasma nitriding, plasma carburizing, or plasma carbonitriding. A hard coating may be applied to the substrate by evaporative reactive ion plating or reactive sputtering of aluminum nitride, silicon nitride, silicon carbide, or nitrides, carbides, or borides of titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, or tungsten.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to magnetic-recording disks, andrelates more particularly to a titanium or titanium-alloy substrate forapplication of magnetic-recording media.

2. Description of the Relevant Art

In a disk file, the most common recording medium is a very flat andsmooth aluminum-alloy substrate disk having both of its surfaces coatedwith a magnetic-recording material such as a ferrimagnetic orferromagnetic oxide powder dispersed in a resin binder or a plated orsputter-deposited thin film of ferromagnetic cobalt alloy.

Typically, an aluminum-alloy substrate of a magnetic-recording disk hassurfaces that are either diamond tool turned on a lathe or ground by asurface grinder. These machined surfaces result in matte finishes.Alternatively, substrate disks may be precision blanked from flatprecision cold-rolled aluminum-alloy sheet or other metal strip whosesurface finish would replicate that of the work rolls used in thefinishing pass of the rolling mill. For example, with work rolls thathave been ground and polished to a mirror-bright finish, a metal stripwith a mirror-bright surface finish would result.

Aluminum is a soft metal and, therefore, can be damaged by a read/writehead impacting the disk too forcefully. To provide for protectionagainst defects caused by impacts of a read/write head, analuminum-alloy substrate is typically first coated with a hard,nonmagnetic material before the magnetic-recording material is applied.A nickel-phosphorus alloy, electroless-deposited from an aqueoussolution, is the hard material commonly used for this application. Inorder for this protective plating to adhere properly to the surface ofan aluminum-alloy substrate, a zincate solution is used to dissolve thesurface aluminum oxides, hydroxyoxides, and hydrous oxides, and toprovide a zinc metal layer by replacement reaction. After coating, thesurface of the nickel-phosphorus-alloy-coated disk is extensively lappedand polished to provide a flat and smooth surface for the application ofthe magnetic layer. This lapping and polishing step is expensive andadds substantial costs to the final disk product.

Apart from the need to lap and polish the disk after application of thenickel-phosphorus alloy, the application of this hard coating presentsadditional difficulties. For example, it is extremely difficult toobtain flaw-free electroless-deposited nickel-phosphorus-alloy coatings.Nodules, pits, and bumps occur in these coatings and such defects causerecording errors.

Another problem is that the electroless-deposited nickel-phosphorusalloy is very prone to recrystallization upon heating, where thenonmagnetic (actually, superparamagnetic) single phase of nickel andphosphorus separates into two equilibrium crystalline phases, namely,nickel, which is ferromagnetic, and nickel phosphide. The resultingferromagnetism renders the media useless for the magnetic-recordingapplication.

Moreover, such a disk is also subject to warpage upon heating because ofstress concentrations at the coating-substrate interface. This warpagealso renders the media useless.

Another problem associated with the use of an aluminum alloy as asubstrate is the added cost for time and labor involved in itsprocessing. Furthermore, entire disks can be rendered useless throughmyriad heat-related effects.

SUMMARY OF THE INVENTION

In accordance with the illustrated preferred embodiment, the presentinvention provides an improved magnetic-recording disk and a process formanufacturing magnetic-recording disks. A hard, flat disk of titanium ortitanium-alloy is the substrate for a magnetic-recording disk.Conventional magnetic-recording materials may be applied to the surfacesof the titanium or titanium-alloy disk substrate either directly orafter first surface-hardening or hard-coating the substrate.

One aspect of the present invention is to provide a substrate precisionblanked from flat precision cold-rolled titanium or titanium-alloystrip.

Another aspect of the present invention is to harden the surface regionof the titanium or titanium-alloy substrate by a hardening treatmentprior to application of the magnetic-coating material. The process usedfor the hardening treatment can be plasma nitriding, plasma carburizing,or plasma carbonitriding.

Yet another aspect of the present invention is to apply a hard coatingto the surface of the titanium or titanium-alloy substrate as analternative to or in addition to the surface-hardening treatment. Theapplication of the hard coating is done by evaporative reactive ionplating or reactive sputtering of a hard material selected from thegroup consisting of aluminum nitride, silicon nitride, silicon carbide,and nitrides, carbides, and borides of titanium, zirconium, hafnium,vanadium, niobium, tantalum, chromium, molybdenum, or tungsten.

A significant advantage of the present invention is that it provides aflat, hard substrate for the deposition of magnetic-recording materials.The present invention reduces the costs involved in the manufacture ofmagnetic-recording disks in both materials costs and waste costs byeliminating the deposition of nickel-phosphorus alloy.

The features and advantages described in the specification are not allinclusive, and particularly, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification and claims hereof. Moreover, it should be notedthat the language used in the specification has been principallyselected for readability and instructional purposes, and may not havebeen selected to delineate or circumscribe the inventive subject matter,resort to the claims being necessary to determine such inventive subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a titanium or titanium-alloy substrate of thepresent invention with a layer of magnetic-recording media applieddirectly to the substrate, with or without a prior surface hardening ofthe substrate. This and other figures herein are not to scale.

FIG. 2 is a side view of a titanium or titanium-alloy substrate of thepresent invention with a hard coating applied to the substrate prior todeposition of a layer of magnetic-recording media.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 of the drawings and the following description depictvarious preferred embodiments of the present invention for purposes ofillustration only. One skilled in the art will readily recognize fromthe following discussion that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles of the invention described herein. While the inventionwill be described in conjunction with the preferred embodiments, it willbe understood that they are not intended to limit the invention to thoseembodiments. On the contrary, the invention is intended to coveralternatives, modifications, and equivalents, which may be includedwithin the spirit and scope of the invention as defined by the claims.

The present invention provides low-cost, high-performance substrates formagnetic-recording applications by providing a substrate disk from aprecision-rolled strip of titanium or titanium alloy. Accordingly, thepresent invention obviates the need for the costly and troublesomeelectroless-deposited nickel-phosphorus-alloy phosphorus-alloy coating,which is otherwise required on an aluminum-alloy disk substrate.

Since the surface finish of the precision-rolled strip becomes a contactnegative of the work-roll finish used in the final pass of the mill, thesmoother the surface finish of the final-pass work rolls, the smaller isthe amount of lapping and polishing required for the substrate disk.Subsequently, both surfaces of the titanium or titanium-alloy substratedisk can be appropriately textured to give a circumferential scratchpattern. This intentional surface roughening serves simultaneously twodesirable purposes: 1) tribologically, to minimize stiction and frictionat the head-to-disk interface; and 2) on the subsequentlysputter-deposited films of the chromium (or chromium-alloy) underlayerand the cobalt-alloy magnetic layer, to provide preferred orientation ofthe crystallites along the circumferential texture lines in thesubstrate plane. For the case of the application of a hard coating priorto the deposition of the magnetic media, the texturing of the disksubstrate surfaces by the abrasive tape or slurry should be more severe(the peak-to-valley heights greater) than ordinarily in order tocompensate for the extraordinary levelizing effect obtained in coatingscharacteristic of the energetic-atom-deposited dense fibrous Zone Tstructure (as shown on the Movchan-Demchishin-Thornton zone-structurediagram) where the energy involved per depositing atom is sufficientlyhigh. References on the subject of coating zone structures include thefollowing: J. A. Thornton, J. Vac. Sci. Technol. 11, 666 (1974); D. W.Hoffman and R. C. McCune in "Handbook of Plasma Processing Technology",S. M. Rossnagel, J. J. Cuomo, and W. D. Westwood, eds., Ch. 21, pp.483-517, Noyes Publications, Park Ridge, N.J. (1990).

As shown in FIG. 1, onto a properly surface-finished substrate disk 11made of a titanium or titanium alloy, a coating of magnetic-recordingmedia 12, composed of chromium (or chromium alloy) and cobalt-alloydouble-layer thin films, is directly applied by sputter deposition.While the magnetic-recording media 12 is shown applied to only onesurface of the disk substrate 11, of course it is conventional andwithin the scope of the present invention to coat both surfaces of thesubstrate with magnetic-recording media. FIG. 1 and the rest of thefigures herein are not to scale.

The surface region 13 of the titanium or titanium-alloy substrate 11 canbe modified to the depth of several micrometers (as much as 100 μm) by ahardening treatment prior to the deposition of the magnetic-recordingmedia 12. The hardening treatment is preferably either plasma nitriding,plasma carburizing, or plasma carbonitriding.

The plasma surface-hardening treatment of the titanium or titanium-alloydisk substrates can be accomplished in the same system designed forevaporative ion plating and described in U.S. patent application Ser.No. 07/771,348, filed Oct. 4, 1991 by the instant inventors, whichdisclosure is hereby incorporated by reference. This system contains anelectrically isolated cantilevered drum planetary substrate holder inwhich six equispaced planet gears, each with its cantilevered shaft,rotate around a nonrotating sun gear, causing the drum housing to alsorotate around the sun gear. The disk substrates through the insidediameter are mounted on (and in electrical contact with) thecantilevered shafts so that they are equispaced along each shaft length.In this way both sides of the disk substrates are simultaneously exposedto the gas discharge. Since the drum planetary and the disk substrateswhen powered are all at the same bias potential, a quasi-hollow cathodeis formed by each facing pair of disk substrates. Thismulti-quasi-hollow-cathode configuration, when high-radiofrequencypowered, results in a gas discharge of high plasma density permitting,for a given power density, substantially higher bombarding-ion currentdensities and lower voltages to be obtained at low pressures incomparison with the well-known simplehigh-radiofrequency-powered-planar-disk-diode configuration.

In the plasma nitriding of titanium and titanium alloys, energeticnitrogen atoms (some of which had been energetic nitrogen ions) from thedissociation of nitrogen molecules in the gas discharge adsorb on thesputter-etch-cleaned surface, diffuse inward at temperatures of 350° to850° C., and hardening is accomplished by the precipitation of very-fineinterstitial alloy nitride grains in the diffusion zone. The nitrogencontent decreases gradually through the depth of the diffusion zone.Deepest into the diffusion zone, the nitrogen atoms are in interstitialsolid solution with the hexagonal-close-packed or body-centered-cubicphases of titanium or titanium alloy. Since the titanium nitride phasewith its double-interpenetrating-face-centered-cubic structure has avery wide range of stoichiometry, this phase forms nearer the surfacebut remains nitrogen-deficient until reaching the surface, where itbecomes near-stoichiometric or stoichiometric.

Prior to beginning the plasma surface-hardening treatment, the disksubstrates (along with the planetary substrate holder) are sputter-etchcleaned by means of, first, an air gas discharge operated anywhere inthe pressure range of 40 Pa (300 mtorr) to 400 Pa (3.0 torr) and at adeveloped dc bias voltage whose magnitude is less than -30 V, measuredwith respect to ground, from the high-radiofrequency (13.56 MHz) powercoupled to the planetary. The air glow-discharge cleaning selectivelyand rapidly removes carbonaceous substances and organic contaminantssuch as oils, greases, and waxes by converting them into volatile gases(CO, CO₂, and H₂ O) which are pumped away. Furthermore, this air gasdischarge provides an extremely effective method for efficiently heatingthe disk substrates by means of the heat input delivered not only by thelow-voltage bombarding ions but also by the surface recombination ofenergetic excited neutral atoms to form molecules. If desired, a gasmixture of oxygen and nitrogen can be used instead of air.

The oxides on the disk substrate surfaces are sputter-etched away anddissociated by means of energetic atom and ion bombardment by hydrogenand argon atoms and ions from a hydrogen-argon discharge operatedanywhere in the pressure range of 40 Pa (300 mtorr) to 400 Pa (3.0 torr)and at a developed dc bias voltage whose magnitude is greater than -30V, measured with respect to ground, from the 13.56 MHz rf power coupledto the planetary. Interestingly, if triode assistance in the form ofthermionic hot-tungsten-filament cathodes is provided in this process,for a given negative dc bias voltage (typically -20 V) on the filamentarray and a given 13.56 MHz rf power coupled to the planetary, themagnitude of the developed dc bias voltage (and hence the bombarding-ionenergy) on the planetary and its substrates can be increased byincreasing the amount of thermionically emitted electrons. Electronemission is controlled by adjusting the temperature of the filament withmore electrons being emitted by increasing the 50 or 60 Hz ac heatingcurrent through the filament.! This behavior observed over the pressurerange of 40 Pa (300 mtorr) to 400 Pa (3.0 torr) is opposite to that ofthe normal triode sputtering source operated over the pressure range of0.1 Pa (0.75 mtorr) to 2 Pa (15 mtorr). Furthermore, the filament arraydepending on its proximity to the disk substrates can provide heating ofthese substrates in addition to that delivered from the energetic ionbombardment by both argon and hydrogen ions and from the surfacerecombination of energetic excited hydrogen atoms to form hydrogenmolecules.

After sufficient time has elapsed to free the disk substrate surfaces ofoxides and to sufficiently heat the substrates, nitrogen is introducedto the gas discharge. Various gases or gas-mixture combinations can beused as follows: (1) nitrogen and hydrogen; (2) nitrogen, hydrogen, andargon; (3) ammonia; (4) ammonia and argon. Since the former twodischarges generate ammonia, and the latter two discharges generatenitrogen and hydrogen, they are essentially equivalent in effect. Theplasma-nitriding surface-hardening treatment of the substrates can takeplace anywhere in the pressure range of 40 Pa (300 mtorr) to 400 Pa (3.0torr). Alternatively, if triode assistance as described above isprovided, then pressures as low as 1.0 Pa (7.5 mtorr) to 4.0 Pa (30mtorr) can be employed.

In an analogous way, the plasma-carburizing surface-hardening treatmentof the substrates can be accomplished with the substitution of ahydrocarbon gas, such as methane, for nitrogen or ammonia. For the caseof plasma carbonitriding, then a hydrocarbon gas, such as methane, isadded to the nitrogen- or ammonia-containing gas mixture.

Since carbonaceous substances and surface oxides on the substrates actas diffusion barriers to the energetic nitrogen and carbon atomsadsorbed from the glow-discharge plasma nitriding, carburizing, orcarbonitriding process, it is essential that glow-discharge sputter-etchcleaning of the substrates and sputter-etch removal of the surfaceoxides take place prior to the plasma surface-hardening treatment. Theplasma, in the presence of a nitriding or carburizing gas, increases themass transfer of nitrogen or carbon atoms, respectively, to thesubstrate surface in comparison with conventional gas nitriding orcarburizing methods. Case depth is still largely controlled bysolid-state diffusion of nitrogen and/or carbon atoms in the titanium ortitanium alloy to form fine-grain precipitates of interstitialnitrogen-containing, carbon-containing, or nitrogen- andcarbon-containing alloys, and of nitride, carbide, or carbonitridephases, correspondingly, a time-temperature-dependent process whichproceeds independently of the plasma.

References on the subject of plasma nitriding, plasma carburizing, andplasma carbonitriding include the following:

T. Bell and P. A. Dearnley, "Plasma Surface Engineering" in "Plasma HeatTreatment, Science and Technology", PYC Edition, Paris, France (1987),pp. 13-53.

M. -B. Liu, D. M. Gruen, A. R. Krauss, A. H. Reis, Jr., and S. W.Peterson, High Temp. Sci. 10, 53 (1978).

C. Braganza, Paganza, S. Veprek, E. Wirz, H. Stussi, and M. Textor,Proc. 4th Int'l Symp. on Plasma Chemistry, S. Veprek and J. Hertz, eds.,Universitat, Zurich, Switzerland (1979), p. 100.

E. Wirz, H. R. Oswald, and S. Veprek, Proc. 4th Int'l Symp on PlasmaChemistry, S. Veprek and J. Hertz, eds., Universitat, Zurich,Switzerland (1979), p. 492.

M. Konuma and O. Matsumoto, J. Less-Common Met. 52, 145 (1977).

M. Konuma and O. Matsumoto,. J. Less-Common Met. 55, 97 (1977).

M. Konuma, Y. Kanzaki, and O. Matsumoto, Proc. 4th Int'l Symp. on PlasmaChemistry, S. Veprek and J. Hertz, eds., Universitat, Zurich,Switzerland (1979), p. 179.

O. Matsumoto, M. Konuma, and Y. Kanzaki, J. Less-Common Met. 84, 157(1982).

T. Shibutami, Y. Kanzaki, and O. Matsumoto, J. Less-Common Met. 113, 177(1985).

T. Shibutami and O. Matsumoto, J. Less-Common Met. 120, 93 (1986).

M. Konuma, "Film Deposition by Plasma Techniques", Springer-Verlag,Berlin, Germany (1992), Ch. 8, pp. 185-194.

As shown in FIG. 2, a 0.5 to 1.0 μm thickness, or other appropriatethickness, layer 14 of a hard coating may be applied prior to thedeposition of the magnetic-recording media 12. The hard coating isselected from the group consisting of aluminum nitride, silicon nitride,silicon carbide, and nitrides, carbides, and borides of titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum,or tungsten, and can be directly deposited on the titanium ortitanium-alloy substrate 11 by evaporative reactive ion plating or byreactive sputtering. Such a hard coating has been disclosed in U.S.patent application Ser. No. 07/771,348, filed Oct. 4, 1991 by theinstant inventors, which disclosure is hereby incorporated by reference.Preferably, the magnetic-recording media 12 is comprised of chromium (orchromium-alloy) and cobalt-alloy thin-film layers.

In addition, the surface region 13 of the titanium or titanium-alloysubstrate 11 can be hardened to the depth of several micrometers (asmuch as 100 μm) by plasma nitriding, plasma carburizing, or plasmacarbonitriding, and then a 0.5 to 1.0 μm thickness, or other appropriatethickness, layer of a hard coating selected from the group consisting ofaluminum nitride, silicon nitride, silicon carbide, and nitrides,carbides, and borides of titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, or tungsten can be deposited onthis hardened surface of this titanium or titanium-alloy substrate byevaporative reactive ion plating or by reactive sputtering prior to thesputter deposition of the magnetic-recording media 12.

The high strengths and low densities of titanium and titanium alloysresult in exceptionally favorable strength-to-density ratios. Titaniumwith its alloys bridges the design gap between aluminum alloys andsteels, particularly the austenitic stainless steels, offering acombination of many of the most desirable properties of each. Thecorrosion resistance of titanium and its alloys is on par with the bestof the austenitic stainless steels. In the passive state each group hasan equivalent position in the electromotive series at the cathodic endjust above silver and graphite, making these groups among the mostcorrosion-resistant of metals. On the other hand, aluminum and aluminumalloys are near the top of this series at the anodic end, making themvery reactive in comparison.

For disk substrates, the most cost-effective choice among titaniumalloys is Ti-3Al-2.5V (ASTM Grade 9)(UNS R56320), corresponding in atompercent to 92.5Ti-5.2Al-2.3V. This alloy possesses high strength andductility in the annealed condition. Further increases in strength occurwith cold work resulting in only a small decrease in ductility andformability, e.g., by the cold-reduction rolling-mill operation. Thiscold reduction is preferably followed by a stress-relieve heat treatmentat 540° C. in an argon atmosphere or in vacuum to protect the metal frompenetration and hence embrittlement by reactive gases.

The 5086 aluminum-magnesium alloy (UNS A95086) is very widely used fordisk substrates by the magnetic-recording industry. A comparison chart(Table 1, below) for some physical and mechanical properties of the 5086aluminum alloy (UNS A95086) and the high-strength titanium alloyTi-3Al-2.5V (ASTMGrade 9) (UNS R56320) shows that, with the exception ofdensity, the latter has all the remaining advantages over the former(Table 2, below). More significantly, with these two materials each inthe annealed condition, the ultimate tensile strength-to-density ratioof the titanium alloy is 1.49 times that of the aluminum alloy, and theyield strength-to-density ratio of the titanium alloy is 2.83 times thatof the aluminum alloy. In the 20%-cold-worked and stress-relievedcondition for the titanium alloy and the 20%-cold-worked condition forthe aluminum alloy, the ultimate tensile strength-to-density ratio ofthe titanium alloy is 1.87 times that of the aluminum alloy, and theyield strength-to-density ratio of the titanium alloy is 2.30 times thatof the aluminum alloy (cf. Tables 2 and 3, below). The melting point ofthe titanium alloy is 1700° C., and the melting point range of thealuminum alloy is 585° to 640° C. Consequently, the heat-distortionresistance is considerably higher for the titanium alloy than for thealuminum alloy.

                  TABLE 1                                                         ______________________________________                                        Comparison of some physical and mechanical                                    properties of 5086 aluminum alloy (UNS A95086) and                            Ti--3Al-2.5 V (ASTM Grade 9) (UNS R56320) titanium alloy                                 Aluminum alloy                                                                            Titanium alloy                                                    5086        Ti--3Al-2.5 V                                          ______________________________________                                        Density      2.66 g/cm.sup.3                                                                             4.48 g/cm.sup.3                                    Electrical resistivity                                                                     5.48 microohm cm                                                                            126 microohm cm                                    Thermal-expansion                                                             coefficient                                                                   (20 to 100° C.)                                                                     23.8 × 10.sup.-6 cm/cm °C.                                                     9.5 × 10.sup.-6 cm/cm °C.             (0 to 650° C.)      9.9 × 10.sup.-6 cm/cm °C.             Melting point range                                                                        585° to 640° C.                                                               1700° C.                                    Modulus of elasticity                                                                      10,300 kpsi   15,000 kpsi                                        (Young's)    71 GPa        103 GPa                                            Ultimate tensile strength                                                     annealed     38,000 psi    95,000 psi                                                      262 MPa       655 MPa                                            20% work hardened                                                                          42,000 psi    132,000 psi                                                     290 MPa       910 MPa                                            40% work hardened                                                                          47,000 psi                                                                    324 MPa                                                          Yield strength 0.2%                                                           annealed     17,000 psi    81,000 psi                                                      117 MPa       558 MPa                                            20% work hardened                                                                          30,000 psi    116,000 psi                                                     207 MPa       800 MPa                                            40% work hardened                                                                          37,000 psi                                                                    255 MPa                                                          ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Ratio property comparison of Ti--3Al-2.5 V                                    (ASTM Grade 9) (UNS R56320) titanium                                          alloy to 5086 aluminum alloy (UNS A95086)                                     ______________________________________                                        Density           1.68 times                                                  Electrical resistivity                                                                          22.99 times                                                 Thermal-expansion 0.40 times                                                  coefficient                                                                   (20 to 100° C.)                                                        Modulus of elasticity                                                                           1.46 times                                                  (Young's)                                                                     Ultimate tensile strength                                                     annealed condition                                                                              2.50 times                                                  20% work hardened 3.14 times                                                  Yield strength 0.2%                                                           annealed condition                                                                              4.76 times                                                  20% work hardened 3.87 times                                                  ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Ratio of the strength-to-density ratio of Ti--3Al-2.5 V (ASTM                 Grade 9) titanium alloy (UNS R56320) with respect to the                      strength-to-density ratio of 5086 aluminum alloy (UNS A95086) for             each of the following conditions:                                             ______________________________________                                        Annealed condition                                                            ultimate tensile strength                                                                        1.49 times                                                 yield strength 0.2%                                                                              2.83 times                                                 20%-Cold-worked condition                                                     ultimate tensile strength                                                                        1.87 times                                                 yield strength 0.2%                                                                              2.30 times                                                 ______________________________________                                    

Where now a 0.050 inch-thickness disk substrate 95 mm (3.740 inch)diameter! of the 5086 aluminum alloy (UNS A95086) is used, a 0.030inch-thickness disk substrate of the Ti-3Al-2.5V (ASTM Grade 9) titaniumalloy (UNS R56320), with its advantage of high strength-to-densityratio, could be used instead, resulting in the same weight for the disksubstrate but a significantly reduced thickness.

Titanium or titanium-alloy strip over the thickness range required fordisk substrates can be precision cold rolled to a tolerance of betterthan ±0.0001 inch (±0.0025 mm) on modern (or modernized) Sendzimer Type1-2-3-4 twenty-high roll-cluster mills. In this mill, the two work rollshave the smallest practical diameter, each of which is backed by twointermediate rolls, and each of these pairs is backed by threelarger-diameter rolls, where in turn each set is backed by fourhigh-capacity bearing assemblies, all mounted in a rugged one-piece casthousing. The extreme rigidity and supporting structure of this millarrangement minimizes work-roll deflection in the horizontal plane aswell as bending in the vertical plane. The small-diameter work rolls inthe reversing mill permit at high speeds the greatest overall (i.e.,with a few passes back and forth) thickness reductions between anneals,easily by 40% with up to 75% being possible. Strip of extremely closethickness tolerance both across the width and along the length ispossible with a Sendzimer mill when equipped with an automaticanticipatory gauge-control system which continuously electronicallyscans and compares the entry and exit strip across the width, and whichsignals a servomechanism controlling the electro-hydraulic screwdownpositioning system for adjusting the mill cluster rolls. A computercontrolling these systems also controls a tensiometer-mechanism systemto provide constant winder tension and a motor-drive system to provideconstant mill speed, plus other compensating adjustment systems. For thefinishing pass of the strip in a reversing mill, the easily removablework rolls may be exchanged for ones that are highly polished to amirror finish. In this way a strip product with a mirror-bright surfacefinish down to less than one microinch can be obtained. Furthermore, aprecision rolling mill can be equipped with an inline tension levelercombining stretcher and roller leveling in a single operation toliterally stretch the metal strip to ultraflatness.

A method of fabricating the disk substrate from the titanium ortitanium-alloy strip is chosen that will not distort and deform the partin cutting the disk outside and inside diameters. Conventional stampingof metals entailing a blanking, punching, and shearing operationtypically does not provide the required precision for magnetic-recordingdisks. The required precision can be met by a forming operation known asprecision blanking, and similarly fineblanking, wherein a high-speedhydraulic punch press, with very little clearance between the punch anddie (less than one-half percent of the piece-part material thickness)and having a dull punch tool edge instead of the usual sharp one, isemployed. The part, being clamped and confined within the die cavity, iscold extruded out of the strip instead of sheared and fractured away.The resulting parts have smooth straight sides with no taper or diebreak.

Alternatively, an electromagnetic-driven very-high-velocity impact punchpress, with a precision die set having near-zero clearance between thepunch and die, can blank parts with smooth straight sides and with sharpburr-free edges and without any die roll. In the usual punchingoperation, the workpiece undergoes three stages: elastic, plastic, andfracture. Very-high-velocity impact punch pressing bypasses the elasticand plastic stages and causes the fracture stage to take place sorapidly that the metal workpiece does not have time to react, and rollededges and burrs do not have time to form. This electromagnetic-drivenvery-high-velocity impact punch press has been designed at LourdesSystems, Inc., Hauppauge, Long Island, N.Y.

Marring of the surface finish on the precision-rolled strip caused fromthe contact by the punch tool and die cavity can be prevented by the useof a strippable plastic coating or adhesive-backed paper on the strip.

These precision blanking procedures are the least-expensive fabricationmethods for cutting the disk substrates from precision-rolled strip.Computer-numerical-controlled laser machining to perform the task ofprecision cutting to finished dimension can also be utilized.Alternatively, abrasive water-jet cutting with the very recentimprovement in its precision can be employed.

Furthermore, titanium and titanium alloys are somewhat difficultmaterials to deal with by conventional metalworking methods. The disksubstrates blanked or cut from precision cold-rolled strip would requirea very minimal lapping step, the smoother the surface finish the smallerthe amount of lapping, and a finishing polishing step. In lapping andpolishing behavior, titanium alloys are much like stainless steels inthat the same techniques, abrasive slurries, and polishing pads can beused and with somewhat more time being required for the former ascompared to the latter. A double-sided planetary lapping and polishingmachine is the preferred approach for this operation.

From the above description, it will be apparent that the inventiondisclosed herein provides a novel and advantageous magnetic-recordingsubstrate composed of titanium or titanium alloy. The foregoingdiscussion discloses and describes merely exemplary methods andembodiments of the present invention. As will be understood by thosefamiliar with the art, the invention may be embodied in other specificforms without departing from the spirit or essential characteristicsthereof. Accordingly, the disclosure of the present invention isintended to be illustrative, but not limiting, of the scope of theinvention, which is set forth in the following claims.

What is claimed is:
 1. A magnetic-recording disk comprising:adisk-shaped substrate composed of titanium or a titanium alloy, whereinthe substrate includes a surface cleaned by sputter etching and thenhardened by plasma nitriding, plasma carburizing, or plasmacarbonitriding; and a coating of a magnetic-recording material overlyingthe hardened surface of the substrate.
 2. A magnetic-recording disk asrecited in claim 1 wherein the coating of magnetic-recording material isapplied by sputter deposition.
 3. A magnetic-recording disk as recitedin claim 1 wherein the substrate is precision blanked from flatprecision cold-rolled titanium or a titanium alloy.
 4. Amagnetic-recording disk as recited in claim 1 wherein the substrate is atitanium alloy comprising Ti, 3% Al, and 2.5% V.
 5. A magnetic-recordingdisk as recited in claim 1 further including a hard material coating onthe hardened surface of the substrate, wherein the hard material coatingis aluminum nitride, silicon nitride, silicon carbide, or a nitride,carbide, or boride of a metal selected from the group consisting oftitanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, and tungsten, and is applied prior to the application of themagnetic-recording material.
 6. A magnetic-recording disk as recited inclaim 5 wherein the hard material coating has a thickness of at least0.5 micrometers.
 7. A magnetic-recording disk as recited in claim 5wherein the hard material coating is applied by evaporative reactive ionplating or reactive sputtering.
 8. A magnetic-recording disk as recitedin claim 1 wherein said surface hardened by plasma nitriding, plasmacarburizing, or plasma carbonitriding has a Vickers harness of at least1000 kg/mm².
 9. A magnetic-recording disk as recited in claim 1 whereinsaid surface is hardened by plasma nitriding, plasma carburizing, orplasma carbonitriding to a depth of at least 15 micrometers.
 10. Amagnetic-recording disk comprising:a disk-shaped substrate composed oftitanium or a titanium alloy; a hard material applied coating overlyinga surface of the substrate, wherein the hard material coating isaluminum nitride, silicon nitride, silicon carbide, or a nitride,carbide, or boride of a metal selected from the group consisting oftitanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, and tungsten, and is applied prior to the application of themagnetic-recording material, and wherein the hard material coating has athickness of at least 0.5 micrometers; and a coating of amagnetic-recording material overlying the hard material coating.
 11. Amagnetic-recording disk as recited in claim 10 wherein the coating ofmagnetic-recording material is applied by sputter deposition.
 12. Amagnetic-recording disk as recited in claim 10 wherein the substrate isprecision blanked from flat precision cold-rolled titanium or a titaniumalloy.
 13. A magnetic-recording disk as recited in claim 10 wherein thesubstrate is a titanium alloy comprising Ti, 3% Al, and 2.5% V.
 14. Amagnetic-recording disk as recited in claim 10 wherein the surface iscleaned by sputter etching and then hardened by plasma nitriding, plasmacarburizing, or plasma carbonitriding.
 15. A magnetic-recording disk asrecited in claim 10 wherein the hard material coating is applied byevaporative reactive ion plating or reactive sputtering.
 16. Amagnetic-recording disk as recited in claim 14 wherein said surfacehardened by plasma nitriding, plasma carburizing, or plasmacarbonitriding has a Vickers harness of at least 1000 kg/mm².
 17. Amagnetic-recording disk as recited in claim 14 wherein said surface ishardened by plasma nitriding, plasma carburizing, or plasmacarbonitriding to a depth of at least 15 micrometers.